The invention relates to a method and a device for controlling the run-down of an induction machine whose stator is connected to a single-phase or polyphase AC main power supply system via an AC power controller which can be controlled by phase gating.
In order to match the electrical power supply of an induction machine to the respective load conditions at that time, in particular during starting and during the run-down, it is known, for example from EP 0 454 697 B1, for a microprocessor-controlled AC power controller or soft starter to be used, which operates on the phase gating principle. This microprocessor-controlled. AC power controller is used essentially for smooth starting and run-down of three-phase asynchronous machines. Three sets of active devices, in general each comprising two back-to-back parallel-connected thyristors, are generally driven by a microprocessor for this purpose.
Such a microprocessor-controlled AC power controller can now be used to ensure that the three-phase asynchronous machine runs down in a manner matched to the respective application by means of suitable control programs. Thus, for example when operating a pump, it is possible to reduce or avoid the surge in water pressure caused when the pump is switched off directly by closing the non-return valve.
For a smooth run-down, the phase gating angle is now increased, and the terminal voltage thus reduced, starting from a phase gating angle of 0xc2x0, that is to say starting from an operating situation in which the entire mains voltage is applied to the stator terminals of the asynchronous machine. However, the squirrel-cage rotors which are generally used initially react to this by increasing the current drawn, but not by reducing the torque. In this operating range, which is referred to as the shunt range, it is virtually impossible to influence or control the operation of the asynchronous machine using a three-phase power controller. Only when the terminal voltage is reduced further as a result of an increasing phase gating angle is the torque characteristic reduced sufficiently that the stalling torque virtually matches the torque required by the load. From this time, the asynchronous machine reacts sensitively such that changes in the terminal voltage or in the phase gating angle result in torque fluctuations, so that a further reduction in the terminal voltage is associated with the risk of the asynchronous machine stopping abruptly.
EP 0 435 038 A2 describes a three-phase motor having a three-phase power controller, and an associated control method for starting and stopping the drive, in which the aim is to achieve smooth starting and run-down, in particular for pump drives. In the method described there, a phase gating angle is increased in steps until the changeover point, which is defined by comparison of the mathematical signs of the voltages, is reached. The triggering time is established by determining the phase shift between the current and voltage, and the motor then runs down controlled in accordance with a control program.
In U.S. Pat. No. 5,684,377, an asynchronous motor having a three-phase controller is controlled in accordance with a flowchart in which the trigger angle is increased or decreased in steps. Smoothing is carried out by calling up stored values for the electronic power at various times and by sum formation, with a maximum power factor still being achieved by increasing or decreasing the trigger angle.
A corresponding situation arises in the publication in xe2x80x9cIEEE Transactions on Power Electronicsxe2x80x9d, 1997, Volume 6, pages 1041 to 1051, in which the trigger angle is likewise varied in steps, and the aim is to achieve maximum efficiency. Finally, GB 21 49 536 A describes the trigger angle for an asynchronous motor being varied in steps, in order to minimize the amount of energy consumed.
Furthermore, EP 0 742 970 B1 discloses a method in which the phase angle is used for controlling the run-down of an induction machine, and the phase angle is itself regulated, after detection of its minimum, to a specific nominal phase angle value.
An object of the invention is to specify a device for controlling the run-down of an induction machine, in which the risk of undesirable, sudden stopping of the induction machine is largely avoided. Furthermore, the invention is based on an object of specifying a device for controlling the run-down of an induction machine.
According to the invention, these and other objects are achieved by the sequence of the method steps alternatively in claim 1 or claim 2. Developments are specified in the dependent method claims.
The invention is in this case based on the idea that the control program which is provided for the run-down of the induction machine and is matched to the respective application should not be carried out until the induction machine has reached an operating state which forms a suitable starting point for the predetermined control program and allows the induction machine to be run down without any disturbances, and without the risk of stopping abruptly.
The measurement of the phase shift between the terminal voltage and the terminal current after each increase in the phase gating angle, as provided according to the invention, provides the information on the operating state of the induction machine required to determine a suitable starting point. The alternative choice, according to the invention, of the minimum phase shift or a predetermined time interval after reaching the minimum phase shift as the operating point results in a suitable operating state as the operating point or starting point for the control program which is provided for a smooth run-down. This procedure is based on the idea that the best starting point for commencement of the predetermined control program occurs when the wattless component is at its minimum. At this time, an operating state is reached in which the induction machine reacts sensitively to a control process. This also ensures that the operating conditions at the start of the control program which is responsible for smooth run-down, are always the same.
In one advantageous refinement of the method, the phase gating angle is increased in equidistant angular steps until the operating point is reached. This can be done particularly easily.
The angular step is preferably  greater than 1xc2x0 and is, in particular, between 1.5xc2x0 and 2xc2x0. This ensures that the operating point can be reached sufficiently accurately, and in a relatively short time.
In particular, the phase gating angle is increased after each main cycle. This allows the operating point to be reached quickly.
In a further particularly preferred refinement of the invention, the phase shift determined in each main cycle is subjected to digital smoothing, in order to avoid incorrect detection of the intended operating point as a result of transient disturbances.
Another object is achieved by a device having the features of patent claim 8. The device for carrying out the methods according to the invention contains a control device for increasing the phase gating angle in steps until a predetermined operating point is reached, and for subsequently controlling the phase gating angle in order to ensure that the induction machine runs down in accordance with a predetermined control program.